This invention relates to the suspension polymerization of vinyl aromatic monomers to form polymer particles in bead form.
Grim, in U.S. Pat. No. 2,673,194, describes the suspension polymerization of vinyl aromatic monomers, whereby an oil monomer is suspended as droplets in an aqueous medium and polymer beads or particles are produced by the use of an oil-soluble polymerization catalyst, such as benzoyl peroxide. The suspending system of the patent is comprised of finely divided, difficultly water-soluble phosphates, and an anionic surface-active agent which serves as an extender. As used in this art, compounds which act to increase the ability of the finely divided phosphate dispersant to stabilize suspensions are termed extenders. The extenders increase the ability of the phosphate dispersant to maintain stable suspensions with a greater proportion monomer and/or polymer in the suspension polymerization medium. Among the extenders listed by Grim are: sodium dodecylbenzene sulfonate, sodium tetradecyl sulfate, potassium stearate, and long chain alkyl sulfonates.
The suspension systems such as that of Grim produce polymer beads having a broad particle size distribution. The individual beads produced in suspension may range in diameter from less than 300 microns to above 2,000 microns with the average bead diameter being dependent upon the amount of suspending agent and extender present in the system. The average bead diameter can be controlled to some extent by varying the parameters of the system such as the ratio of suspending agent to extender or the ratio of suspending agent and extender to monomer. Although the average diameter size is changed through such variations, nevertheless, beads will be produced whose particle size range is broad.
For commerical reasons, it is highly desirable in this art to control the average particle size range within relatively narrow limits. This is especially true where the polymer beads are to be impregnated with a volatile expanding agent to produce polymer beads which will, upon heating, expand, e.g., in a mold, to fill the mold cavity and produce fused, expanded polymer articles.
One of the major uses of expandable polymer beads, particularly polystyrene beads, which are produced commerically by suspension polymerization, is in insulation applications such as insulation board. In manufacturing insulation board, it is common to mold a large (e.g. 4 ft. .times. 8 ft. .times. 20 ft.) billet of expanded polystyrene and subsequently cut the billet into slabs one or two inches thick for ultimate use as an insulation board. In preparing the large billets from expandable polymer beads, the beads are first pre-expanded to form non-fused, partially expanded beads having a bulk density of from 0.8-1.2 pounds per cubic foot. The partially expanded beads are then charged to the billet mold, and heat, usually in the form of steam, is applied to fully expand the beads whereby they fill the mold, fuse, and form the billet.
In the billet-molding application, it is especially critical that the expandable polymer beads be relatively large and substantially spherical.
If the beads are too small, then the expandable beads at the outer surface of the billet mold will fuse too soon, thereby excluding steam from the center of the mold. The resulting billet, therefore, has a center of unfused, expanded beads which is, of course, highly undesirable since any unfused portion of the billet is useless.
Hohenstein et al, in U.S. Pat. No. 2,652,392, reduced the amount of small particle-size beads formed by adding a water-soluble persulfate as extender for the calcium phosphate stabilizer.
Wright, in U.S. Pat. No. 3,631,014 shows that narrow distribution of bead sizes larger than 300 microns in diameter can be produced by the addition of at least 0.0003 percent by weight based on monomer of sodium bisulfite as sole extender for the phosphate.
Wright, in U.S. Pat. No. 3,649,610 shows that narrow distribution of bead sizes larger than 750 microns in diameter can be produced by the addition of at least 0.01 percent by weight based on monomer of certain terminal vicinal hydroxy-keto compounds as extender for the phosphate.
Wright, in U.S. Pat. No. 3,755,282, shows that narrow distribution of bead sizes larger than 1200 microns in diameter can be produced by the addition of 0.0001 percent by weight of alpha, beta-unsaturated carboxylic acids as sole extender for the phosphate.
Thus, all these methods show ways of affecting the average bead particle size in suspension polymerizations by varying the extender for the phosphate. The phosphate itself can lead to variations in the bead particle size. The efficiency of the phosphate has been found to vary with the age of the lot of phosphate and with the individual lot used.